In cold regions, ice storms cause melted snow, ice and frost to accumulate on conductors of transmission lines. The weight of such icy deposits adds to the wind charge and increases the mechanical tension in the conductors, which may cause the cables to sag excessively or break. These problems are particularly significant with regard to single conductor cables and to bundles of such conductors. Bundles of conductor cables comprise a plurality of individual conductors, usually spaced-apart or separated from each other by means of spacers to provide better electrical characteristics.
A limited number of de-icing methods have been developed and used by electric utilities to counteract and prevent the formation of ice on high voltage power line cables. Conventional practices for avoiding or limiting the formation of ice generally comprise mechanically shocking the conductors or passing high or short circuit currents trough the lines. Mechanical methods involve miscellaneous de-icing tools and rollers which most of the time are used after the ice storms. These methods are generally used for ad-hoc de-icing operations in the field and require very expensive workmanship. High and short circuit current methods consist of heating lines sufficiently to prevent heavy ice formation and to cause ice already present to fall off. For this purpose, either the normal supply transformers are used in special connection, or the current from special transformers replaces the current of the line itself. Such practice has certain inherent disadvantages, particularly to the needed current load, which requires an enormous amount of power to raise the temperature sufficiently to cause the ice deposit to melt at the cable interface and then shed. Often, the current supplying capacity of the transmission system is insufficient to accomplish this task and excessively large and expensive auxiliary transformers are then required. This method is also inconvenient in that the line operation must be interrupted to carry it out.
Another limitation of the already developed thermal methods mentioned above comes from the phase stability of the line, which shall be maintained and assured during its operation. For example, with actual 735 kV power lines involving bundles of four 35 mm conductors, load current per phase shall be kept below 1760 A to maintain phase stability, so that the maximal current circulating into each sub-conductor cannot exceed 440 amperes. This intensity is much too low for removing any ice from these conductors by Joule effect.
Known in the art are U.S. Pat. Nos. 4,119,866 (GENRIKH et al) 4,085,338 (GENRIKH et al) 4,135,221 (GENRIKH et al) which disclose thermal de-icing methods using high current or short circuits. These patents describe methods applicable to de-icing of high voltage lines by short-circuiting the electrical conductors in which a DC current is injected. This operation however requires that the line be interrupted before being connected to the special auxiliary transformers and rectification units of the DC power supplies.
As can be seen, even if the effectiveness of power line de-icing by using high currents and short circuits is well recognized, all the methods developed up to now have the disadvantage of being very expensive, requiring large power equipment, and necessitating an interruption of the current circulation in the line during the de-icing operation, thereby interrupting service to customers.
Also known in the art are U.S. Pat. Nos. 3,316,344 (KIDD et al) and 3,316,345 (TOMS et al), which both disclose an autonomous de-icing method consisting of heating overhead conductors by wrapping them with sets of wires and rods made of a ferromagnetic alloy material having a low Curie Point. In this particular method, the heat is generated by Foucault currents induced within the ferromagnetic wires by the AC current of the transmission line. The weight of the wires is a general function of the temperature and wind conditions. Even if these electromagnetic heating wires operate autonomously with great reliability, since there are no mobile parts, and without any service interruption, they have the major disadvantage of consuming energy all year long because they continually dissipate heat, even if the heating decreases by 20% at temperatures over 20.degree. C. (Fujukura, 1987). For that reason, this system cannot be used economically for de-icing a whole line; its actual use is limited to melting wet snow of very short sections of line, as those at cross-roads. For a de-icing system of this type to be effective under severe freezing rain conditions (-5.degree. C. and 15 m/s), it would require about ten times more energy than the existing system used for melting snow.
U.S. Pat. No. 2,870,311 (GREENFIELD and al) discloses a system involving a specially built cable which can be de-iced by Joule effect. For that purpose, two electrical circuits have been integrated within the cable: one with a low electrical resistance, which is used under normal operation, and the second with a greater resistance, which has the ability of de-icing the cable by Joule heating. During an ice storm, the low resistance circuit has to be changed to the high resistance one. This circuit transfer requires special equipment. In addition, this system requires the replacement of existing cables by new ones which are completely different and thus more expensive to manufacture. The circuit transfer also requires a brief interruption of the line current. To our knowledge, this system has not yet been applied to a transmission line.
Finally, U.S. Pat. No. 2,797,344 (PEIRCE) discloses an apparatus and a method for de-icing two coaxial electric cables. Peirce teaches providing the outer cable with a gap and an electrical bridge to fill this gap. When the ice accumulation on the line reaches a certain value, the electrical bridge is mechanically triggered to free the gap, thereby interrupting the current circulation in the outer cable. The two cables being electrically connected, the entire load of the line is transferred to the inner cable, which generates enough heat to de-ice both cables. The apparatus and method according to this patent are however only applicable to coaxial conductors having appropriate electrical properties.